1. Field of the Invention
The present invention relates to a steel production facility and a method of uninterrupted or at least cyclical steelmaking in said facility, at the least comprising an electric arc furnace (EAF) for uninterruptedly or at least cyclically melting charge materials like shredded scrap-iron pieces in particular.
2. State of the Art
Steel can on the one hand be made from iron ore and pig-iron via the route of blast furnace and converter. However, with respect to energy efficiency, it is on the other hand more advantageous to produce steel mainly by melting pieces of scrap-iron in the electric arc furnace, which is still the most used charge material worldwide for electric arc furnaces.
Steelmaking in the Electric Arc Furnace (EAF):
In the common electric arc furnace process, electrical and chemical energy is used for cyclically melting the charge material. During this process, a huge part of the total energy is transformed into thermic energy that entails the melting of the inserted material. The heat above the electric arc, which burns between the electrode and the charge material, is transferred to the charge material mainly by radiation.
As in every melting process, an oxidic mass is produced in this process, the slag, which due to its lower specific weight floats on top of the molten steel and to which are transferred the unwanted secondary elements separated from the molten material.
A cyclically melting process nowadays normally takes between 30 and 60 minutes (depending on the transformer and the charge material). After the melting process there follows the so-called tapping, which means that the liquid steel is tapped into a steel ladle and in the course of the secondary metallurgy, is refined and cast with further alloy additions according to the customers' demands. The time between two steel taps is in the following defined as cycle of a melting process.
For the purity degree of the steel and its casting quality it is important, that during the tapping into the ladle as little slag as possible or no slag flows along with it. In order to avoid that, it has been common practice to this day, at first and before the tapping of the fluent steel, to discharge the slag out of the furnace into a slag bucket and to cast the molten steel separately thereof into the ladle.
Older electric arc furnaces are designed to provide, for the separate discharge of slag and steel, two openings arranged at the furnace wall at opposite sides and on different levels, which openings can usually be closed and controlled by means of a plug system or in a more modern way by means of a slide system. For the purpose of a reliably separated discharge of slag and steel, the complete furnace was pivoted to the respective opening for discharge, which means, at first to a slag off position between 10° and 15° towards the slag discharge opening arranged on a higher level and then, to a tapping position of circa 45° towards the steel tap opening arranged on a lower level.
In order to make it possible to at least partially reduce or simplify demanding pivoting mechanisms for the furnace, it was suggested to reposition the steel tap opening from the lateral wall of the furnace to the bottom of the furnace. Like in all cases of flowing in and flowing off below a liquid surface, there can occur vortexes, which due to their circular or spiral downward movement may have the unwanted effect of dragging along pieces of slag.
To avoid that, it is commonly known that a certain rest of slag and/or a certain steel sump, remains as minimal quantity (circa 15% of the volume of the furnace) in the furnace, which quantity is at the same time conducive to the undisturbed continuation of the cyclically following melting reduction.
Since then, it has become a common feature of modern electric arc furnaces, that the steel tap opening is arranged at the bottom of the furnace between the center of the furnace and the wall of the furnace. The so-called eccentric bottom tapping (EBT) has the effect, that the furnace now needs to be inclined a few degrees only (up to 15° degree maximum), that means, at first, for the discharge of the slag, towards the slag discharge opening still arranged at the wall of the furnace, and then, for the tapping of the liquid steel, towards the steel tab opening eccentrically arranged at the bottom of the furnace. This implies advantages with regard to the volume and the cooling of the furnace. Moreover, the problem of slag running along is reduced by this type of steel tapping.
If—as it is usually the case with modern electric arc furnaces—during the melting process, especially by means of so-called refining lances, there is added oxygen (“refining”) and carbon, at the surface of most steel types emerges a slag foam, which mainly consists of enclosed gases.
Even foam slags can be slagged off in the classical way. However, it is very common practice, to arrange the slag discharge opening at a level of height related to the melting bath which is defined or definable by a slide system, in such a way that an overrun of foam slag can drain off according to the overflow principle, thus after exceeding a capacity limit, as soon as the melting bath has reached a certain level, whereby breaks caused by slagging off during the melting process are advantageously avoided, at the end of which the classical steel tapping via EBT takes place again.
For the purpose of reaching a productivity as high as possible for the electric arc furnace, it has been always attempted up to now to melt as quick as possible, to add as much electric energy as possible during the entire melting period and to make breaks or in-between intervals without energy supply as short as possible. This is, because the shorter the interval between two tapping processes is, the more flexible is the steel mill regarding its producing structure. Contributing to this are, among other things, also the 800 mm electrodes which were put on the market a few years ago, which allow higher intensities of current and faster tappings. Thus, in modern electric arc furnaces, an electric arc with an intensity of up to 140.000 Ampere makes up to 200 tons of steel scrap melt. At the electric arc furnace there are temperatures of up to 3.500° C. and in the steel bath of up to 1.800° C.
Slag off and tapping off periods up to this day however lead to the typical, cyclical breaks in the supply of electricity, charge materials and additives like fine-grained solid materials and therefore cause the typical, discontinuous process-run of an electric arc furnace.
Feeding of Electric Arc Furnaces (EAF):
Scrap-iron, as a recovered raw material, is available in many different shapes and configurations. According to its properties and to the demands of the melting process and the desired steel qualities, the discarded iron and/or steel junk (scrap) undergoes different measures of preparations. The price of scrap-iron is changing frequently not only due to the market situation, but also due to the final physical and chemical properties of scrap-iron.
In steelmaking the charge material is selected in accordance with the final product which is to be produced. For simple steel grades normally the cheapest scrap-iron is used. This scrap-iron is usually discarded prepared iron and/or steel junk (scrap). The density of this scrap-iron is normally less than 0.4 kg/dm3. Three to four scrap-baskets are normally needed to charge the furnace-shell of an ordinary electric arc furnace. When, as necessary for this, the furnace-roof is opened by pivoting for charging the furnace-shell, energy losses between 15 to 20 kWh/t of steel have to be expected. The interruption of the melting process by normally 4 to 7 or more minutes per each tapping off of slag and steel plus charging with scrap-baskets reduces the productivity and increases the electrode consumption due to additional oxidation of electrodes.
To increase the density of the charge material it is well-known to press the scrap-iron. After pressing the scrap-iron into bundles the density is increased and consequently fewer scrap-baskets have to be charged. However, the melting process has still to be interrupted for the charging.
However, it is only the initial charging of scrap-iron, as the case may be, with Direct Reduced Iron (DRI) and/or Hot Briquette Iron (HBI) and slag formers into the electric arc furnace, which creates the conditions for melting those charge materials and for forming a molten metal bath, which is covered by molten slag.
Recovery of Heat and Energy (Generally):
The possible air pollution by gaseous and dust-laden substances is considered the most essential environmental problem implied in steelmaking from primary raw materials (mostly ores or pellets made from ores). The metallurgical processes are potential sources for the emission of dust and metals from furnaces, converters and from the transport of molten metals.
Furthermore, the energy consumption and the recovery of heat and energy are important aspects of the production of iron metals and steel. They depend on the efficient use of the energy included in ores and admixtures, on the energy demand of the process levels, on the type of energy that is used and on the method of energy supply as well as on the use of efficient methods for heat recovery.
Thus, for the route of furnace and converter was suggested (see GB 958731 A=CH 415 709 B) to, directly or indirectly via a steam production device, feed a turbine with process-exhaust, which turbine powers a generator, the energy of which is used for powering turbo blowers or cowpers of the blast furnace.
Elsewhere it was suggested to use the power generated in particular indirectly via a steam production device from the process-exhaust of a rotary furnace for predrying brown coal (see. GB 1241715 A=DE 19 27 558 A1), for producing oxygen, for feeding the power grid or for powering so-called submerged arc furnaces (see U.S. Pat. No. 4,551,172 A=EP 0 139 310 A1), however, submerged arc furnaces are not used for steel production but for the reduction of slag in order to recover metallic components.
Recovery of Heat and Energy (by EAF):
During the production of steel from secondary raw materials like scrap-iron in the electric arc furnace, gaseous and dust-laded substances are emitted as well; and thus the most essential environmental problems are related to the emissions as well.
Well-known among the methods for heat recovery from the hot process-exhaust (furnace top) of an electric arc furnace is particularly the use of exhaust for drying and preheating of charges (see for example U.S. Pat. No. 3,565,407 A=DE 18 04 098 A1 as well as U.S. Pat. No. 5,153,894 A=EP 0 385 434 B1). There has been, however, no further use of this heat to this day. Efficient dedusting plants and filters are therefore necessary.
Recovery of Electrical Energy (by EAF):
An electrical power recovery before or after the cleaning of process-exhaust (furnace top) is also possible in most cases of electric arc furnaces, but the local situation is very important, like e.g. if the electric arc furnace is operated in mini-mills (compact-mill) and foundries and there is no possibility to use the recovered energy other than to feed it into the national power grid, which is already subject to the danger of unwanted system perturbations resulting from the procedurally determined irregular burning of an electric arc. Therefore, there are always high demands on the power supply of an electric arc furnace.
As electric arc furnaces however have so far functioned as fed-batch-process, which means that they are cyclically fed with batches of application materials like pieces of scrap-iron, Direct Reduced Iron (DRI) and/or compressed Hot Briquette Iron (HBI), the temperature of the process-exhaust undergoes cyclical changes. To compensate for that, in the context of a case study of the ZERO EMISSIONS RESEARCH IN AUSTRIA (ZERIA), an initiative on behalf of the Federal Ministry of Transport, Innovation and Technology (BMVIT) and of the WIFI of Austria (see http://zeria.tugraz.at/index.php3?lang=de&sel=09Fallstudien/01Marienhütte) it is suggested for the steel mill “Marienhütte” to control the exhaust temperature by means of an additional gas burner. For this, complex means of measurement and control have to be provided. Moreover, the use of gas burners for stabilization of the exhaust temperature has the disadvantage of additional use of primary energies and of costs implied therein.